1. Field of the Invention
This invention relates to a rotary percussion drill and more particularly to a hydraulically operated drill that combines the effects of rotation and percussion where the percussion is controlled by a servovalve at variable frequency and stroke.
2. Description of the Prior Art
Rotary percussion drills which combine rotational and percussive forces to produce high torque and high speed for rock drilling in underground mines are well known in the art. U.S. Pat. 3,547,206 discloses a hydraulically operated reciprocating piston for imparting percussion forces to a drill rod nonrotatably connected to a drill rod support member. A multivalve control device opens and closes the valve ports which supply pressurized fluid to the cylinder housing the piston. The timing device controls the opening and closing of the valve, thereby controlling the rate of percussion of the piston. A timing device for supplying fluid under pressure to the cylinder also functions as a positive displacement to displace excess fluid supplied to the cylinder.
U.S. Pat. No. 3,467,207 teaches an extensible drilling machine having both rotary and percussion drive means provided in a drill head. A hydraulic motor provides both the rotary action and the percussion action. The rotary percussion drill has a motor for driving four matched eccentric weights. The weights apply thrust forces to the drill head in a direction parallel to the longitudinal axis of the drilling implement. Thus, linear forces are imparted to the drilling implement, and the eccentric weights rotate in pairs in opposite directions. The weights are in forward and in rear positions at the same time enabling the forces to combine and either add or subtract from the force already being applied as the thrust.
With conventionally known rotary percussion drills of the valve type and primarily those which are actuated by pneumatic pressure, the rate at which the drill bit may be advanced is limited by the opening and closing of the valves and further by the inherent losses that take place in a pneumatic system. On the other hand, rotary percussion drills that utilize eccentric weights to impart percussive forces experience substantial vibration that shortens bit life and reduces drilling rates. Furthermore, the vibration problem is compounded by excessive noise levels encountered when the drill is operated in the percussion mode.
There is need to provide hydraulically operated rotary percussion drill that in the percussion mode reduces the vibration and noise level and provides variable stroke and frequency for drilling in hard rock at drilling speeds up to 3 and 4 feet per minute.